An industrial reciprocating gas compressor is a positive-displacement machine wherein the gas to be compressed is trapped in an enclosed space and then squeezed into a small volume by the action of a piston moving inside a cylinder. The gas is compressed to a pressure sufficient to overcome the discharge pressure plus the spring tension holding the discharge valve closed, at which time the discharge valve opens and allows the compressed gas to leave the cylinder. Because of the nature of the reciprocating piston, compression ceases at the limits of its stroke, the discharge valve again closes due to the action of the springs on the valve, the piston reverses direction, and the small amount of gas remaining in the cylinder expands, increasing in volume and decreasing in pressure, until the inlet pressure is higher than the pressure inside the cylinder plus the spring tension holding the inlet valves closed. The inlet valves then open, allowing gas to flow into the cylinder. At the opposite limit of the piston stroke, the inlet valve also closed due to the action of the springs acting on the valves, the piston again reverses direction, and the compression cycle begins anew.
The rate of pressure rise with respect to piston position in the cylinder, the exact moment of valve actuation, actual pressures attained, and other information concerning the compression, discharge, re-expansion, and inlet events taking place in the cylinder have long been recognized to be of value to engineers in assessing the operating condition of compressors. The first instrument used to record such information was a mechanical device which comprised a stylus attached to a pressure indicator and a rotating drum which was activated in proportion to the movement of the piston by a string attached to the crosshead of the compressor. The instrument was attached to the cylinder with a three-way valve, and sensed pressures inside the cylinder via passages (indicator ports) drilled during manufacture of the compressor. Compressor speeds were limited to 300 rpm or less.
The state of the analyzing art advanced during the 1960s with the introduction of the BETA 100, an oscilloscope device that utilized pressure transducers to sense pressure through the indicator ports, and determined piston location from the angle of the crankshaft of the compressor. Further refinements of such electronic analyzers have been based on the principles of the early devices, and now include displays of pressure/volume or pressure/time, automatic calculation of horsepower consumed by the cylinder, and volumetric efficiency of the cylinder, as well as analysis of each valve event. Data from displacement transducers (for determining compressor rod `run-out`), vibration transducers, and ultrasound detectors (for analyzing the behavior of individual valves) are routinely superimposed on the pressure-volume trace to pinpoint operational problems and to determine the need for maintenance. Data from the analyzer may be sent to a personal computer for the automatic generation of reports.
Significant savings in operational expenses and maintenance costs are attainable if the information is analyzed at routine intervals. There was a gap between the use of the drum-and-stylus instruments and the modern electronic instruments, however, and manufacturers stopped providing indicator ports in cylinders during the 1960s, 1970s and early 1980s and usually did not list indicator ports as an option when ordering a new machine. As a result, many compressors currently in operation do not have indicator ports and therefore cannot be analyzed properly.
Detailed description of a more or less contemporary monitoring system for reciprocating piston machines, including the use of pressure transducers, is given by Wiggins in U.S. Pat. No. 4,456,963. Other references to the use of pressure monitoring are given by Rice in U.S. Pat. No. 4,111,041 and Abnett et al in U.S. Pat. No. 4,325,128. However, I am not aware of a reference in which the pressure transducers are able to sense the cylinder pressure through the center bolt or other center port of a valve such as a discharge valve, as I do.
Installation of indicator ports after manufacture conventionally involves complete disassembly of the compressor and tedious machine shop work to locate and drill the ports. The compressor must remain out of service for extended periods of time with lost production costs accumulating. Often, the cylinder casting does not have provisions for adding an indicator port, and installing one entails penetration and sealing the water jacket surrounding the cylinder. Many times, cylinder liners do not have indicator ports or the indicator ports in the cylinder liners do not align with indicator ports in the cylinder. Without indicator ports, much of the intelligence necessary for analysis is lost. There is clearly a need for a simple and effective way to equip compressors with indicator ports for monitoring compressor cylinder condition and performance.